Method for making iron powder



Patented July 25, 1950 METHOD FOR MAKING IRON POWDER Max F. W. Heberlein, Rahway, N. 3., assignor to The American Metal Company, Limited, New York, N. Y., a corporation of New York No Drawing. Application January 22, 1948, Serial No. 3,815

10 Claims.

This invention relates to a novel method for making iron powder. More especially, the present invention provides an improved electrolytic method for making porous, malleable and semicrystalline iron powder suitable for use in the manufacture of compacts.

I am aware that methods for making iron powder are already known and, further, that it is known that iron powder can be made electrolytically, i. e., by causing the deposition of iron on an electrode forming part of an electrolytic cell. All methods already known, however, as far as I am aware, require the operating conditions to be such that oxidation is avoided during the electrolysis and during the subsequent Washing and drying step. Indeed, prior methods even suggested the introduction of reducing agents, such as sugar and urea, in order to inhibit oxidation of the metal deposited on the cathode during the electrolysis.

By my present invention I obtain iron powder, suitable for use in compacts, by means of an electrolytic method involving conducting the electrolysis under oxidizing conditions. Furthermore, the subsequent washing and drying operation is also conducted under similar conditions. The iron powder obtained in accordance with my novel process can be employed for purposes for which such material is normally used and possesses properties which make it especially suitable for use in the manufacture of compacts, the term by which the powder metallurgy art refers to briquettes obtained by pressing.

Generally speaking, my novel process is conducted by employing as an electrolyte a substance I which has an oxidizing effect. Advantageously, electrolytes which ionize readily are preferred. As an example of an electrolyte which I have found to give excellent results in accordance with my process I mention ferrous perchlorate in the form of an aqueous solution thereof.

In carrying out my novel process I prefer to employ rolled soft sheet steel as the anode. Especially desirable results are obtained by treating this anode material with a 10 per cent solution of sulfuric acid prior to its use in my process. Other materials may be employed as the anode, provided they, like rolled soft sheet steel, easily give good electrical contacts and uniform anode corrosion.

Materials which exhibit little or no corrosion under the conditions of the electrolysis can be employed as the cathode. I have found stainless steel to be a preferred material for this purpose.

The preferred electrolyte for use in my process is the aforementioned ferrous perchlorate. A convenient way to prepare it is to dissolve iron in perchloric acid of about 20 to 25 per cent con centration and then dilute the resulting solution. of ferrous perchlorate until the iron content is preferably approximately grams per liter. If desired, however, the concentration of iron (Fe) may be somewhat lower or somewhat higher, within the range of about 25 to grams per liter. From the operating standpoint I have found it to be desirable, regardless of the concentration employed, to hold the Fe substantially constant during the electrolysis. Means of ac complishing this purpose are known to the art and therefore need not be stated here.

While my invention is not limited to the use of any particular material as the cell container I prefer, on the basis of practical considerations. to employ wooden cells, though other containers such as glass may be used. Wooden containers not only serve the purpose from the technical standpoint but also stand up well during long continued plant operations, especially if they are painted, inside and outside, with a suitable acid resisting paint. Such paints which in addition to being acid-resistant also withstand elevated tern-- peratures in the wet and dry state are available on the market.

The conditions under which the electrolysis is conducted may be varied within somewhat wide limits. It is believed that understanding of this invention will be furthered by discussing these variations in conditions in connection with a spe cific example. It will be understood, however, that the following detailed example is not to be construed as limiting the scope of my invention but is for the purpose of illustrating my inventicn.

In accordance with the foregoing, I prepare an electrolytic cell as follows:

A wooden cell of 10 liters capacity and dimensions of about 6 inches by 9 inches by 12 inches is painted on the outside and inside with an acid-resisting paint. Rolled soft sheet steel having dimensions of 4 inches by 8 inches is used as the anode after it has first been pickled for A; to hours in a 10 per cent aqueous solution of sulfuric acid. As the cathode, a sheet of stainless steel having dimensions of 5 inches by 9 inches is used. The electrolyte was prepared by dissolving iron in a 20 to 25 per cent aqueous solution of perchloric acid and adjusting the iron content to 35 grams per liter.

.each time with 1000 grams of water.

The electrolytic cell so constructed is connected to a source of electrical energy and the electrolysis is begun. Soon the chemical composition of the electrolyte attains equilibrium at a pH of approximately 4.0 to 4.5. The pH may be allowed to drop to about 3.5 Without noticeable adverse effects. However, it is undesirable to permit the pH to go under 3.0, for if it does there forms a deposit of hard material on the cathode, the deposit tending to break, curl up and short circuit the cell. A pH of above 4.5 is not desirable, because the precipitation of ferric hydroxide increases to form such amounts that it becomes difficult to conduct the desired electrolysis.

Very little circulation of the electrolyte is called for, since it serves only to prevent stratification.

A current density of about 35 amperes per square foot of cathode area has been found to give optimum results. Maximum current efiiciency and minimum cell voltage attend the use of this current density in the example here given. Higher current densities, up to 50 amperes per square foot, have yielded excellent results. On the other hand, a current density of as low as 23 amperes per square foot reduces the current etfi ciency markedly.

When a current density of 35 amperes per square foot is used, a cell voltage of about 1.4 to about 1.6 volts can be maintained for an extended period, e. g., several days. Under these current conditions the temperature of the electrolyte rose to about 120 F. without the application of any external heat. A number of cells, which were connected inseries, were operated for a duration of about 3 months with only minor adjustments relative to pH and Fe content of the electrolyte.

During the operation of the cell about 6 per cent of the dissolved anode iron does not deposit on the cathode, but instead settles as ferric hydroxide at the bottom of the cell. Accordingly, there is a slow decrease in the iron content of the electrolyte as the electrolysis progresses. It is advisable to remove the deposit of ferric hydroxide, at intervals of B to 8 weeks, for example. The isolated ferric hydroxide can then be dissolved in perchloric acid with some metallic iron, to form a neutral solution of ferrous perchlorate. This solution can be added to the electrolyte in the cell to adjust for losses due to drag-out or spillage, and also to adjust for the decrease in iron content caused by the aforementioned precipitation of ferric hydroxide.

Under the indicated preferred conditions, iron deposits at the cathode as a solid but very friable and spongy sheet. It is desirable to strip this deposit ofi frequently, for example, every 24 hours. If permitted to build up for longer periods, the deposit would be so thick that it would be more difficult to handle in the subsequent operations, and, furthermore, the deposit tends to become less friable with increasing thickness.

If stripped as recommended above, the deposit breaks readily into pieces of convenient size while being stripped from the stainless steel cathode.

'The stripped and broken deposit, obtained as indicated, is washed twice, each time with a weight of water equal to its own weight. In other words, each kilogram of deposit is washed twice, This has been found sumcient to produce a stable and nonpyrophoric iron powder of high purity.

The washed deposit .is dried by the application of heat. This can be done in any known and convenient manner as, for example, in a steam drier, on a hot plate, or even over an open flame. Considerable oxidation of the deposit takes place during the drying operation. The amount of iron oxide (FezOs) present in the dried deposit varies from about 10 per cent to as high as about 20 per cent, being about 12 to 15 per cent on the average.

The drying operation eliminates any residual perchloric acid (HClOi) still present after the indicated Washing operation. Consequently, the properties of the final product are not adversely aiiected by perchloric acid.

The dried deposit can be ground in any convenient manner. A red mill is advantageous for this purpose, though other known grinding means, for example, those employing a hammer mill, yield good results. The grinding operation, desirably, is conducted so as to yield a ground product a substantial proportion of which will pass through a mesh screen.

The ground material is then screened, the material which does not pass through the 120 mesh screen being returned to the grinder, Where it may be ground per se or in admixture with a new batch of the dried deposit obtained in accordance with the foregoing.

The ground and screened material is then reduo-ed. This operation is advantageously performed in an atmosphere of hydrogen at a tom perature within the range of about 1150" F. to about i400 R, for a period of 3 hours. Prefer-- ably, the temperature may be so controlled as to increase at a substantially constant rate from about 1150 F. at the start to about 1400" F. at the end of the reduction period.

In conducting the reduction, hydrogen gas is passed through a furnace containing the material to be reduced in such a manner that a pres sure slightly over atmospheric is maintained inside the furnace. The exit gas may be burned or, of desired, it may be recycled; in the latter case the exit gas must be treated to remove moisture prior to re-cycling.

The iron, which at this point is substantially completely reduced, is removed from the reduction furnace, normally in the form of agglomerated cakes. These are broken into small pieces and disintegrated into powder in a grinder such a hammer mill.

The iron powder obtained in accordance with the foregoing is light gray in color and has a bright metallic luster. Its physical properties and chemical purity are shown by the following data which are typical for the products obtainable in accordance with the novel process of this invention.

Mini- Maxi mum mum Loading Weight .-grams per 0. c 1.93 3. 47 Flow 1 minutes. 45 5. 0 Screen Analysis:

+100 Mesh nil trace Mesh per cent. nil 0. 6 +200 Mesh--- 29. 5 +250 Liesh l0. 0 24. 6 98. 3

Three (3) lots were prepared by blending a number of individual batches. They had the following characteristics:

I II III Loading Weight 2. 86 2. 84 2. 96 Flow 1 "seconds" 36. 8 74. 4 37. 4 Screen Analysl +100 Mesh trace nil nil +150 Mesh trace l 1 +200 Mesh 2. 3 4. 4 5. +250 Mesh- 2. 5 2. 5 2. 6 +325 Mesh- 21. 0 l2. 5 18. 0 -325 Mesh 74. 2 80. 5 74. 3 Chemical Analysis:

C u per cent .015 .08 .10 S1 (as S1O ,do .03 .03 .02 C do". .04 .05 .03 Cr and Al do. trace trace trace Mn do. .186 .15 .10 Ni .do .005 .005 .002 Fe "do 99. 01 99.22 99. 23

l 1 Time for 50 grams of sample to flow through the orifice of a Hal Flowmeter.

In the data on screen analysis the characters and before the mesh size means, re spectively, that the particular portion is too coarse to pass through a certain mesh size screen, or it is of such fineness, that it will pass through. it.

All concentrations given herein are to be taken to mean by weight, unless otherwise specifically stated.

While the process of this invention has been described in detail for purposes of illustration as aforementioned, it will be understood by those skilled in the art that minor modifications may be made herein without departing from the spirit of this invention. It is my intention not to be limited by the foregoing detailed description, for I intend to secure by this patent all such modifications coming within the scope of the appended claims.

I claim:

1. In a process for making iron powder, electrolyzing an aqueous acid solution consisting essentially of ferrous perchlorate dissolved in water maintained at a pH of at least 3 and electrodepositing a friable spongy mass of iron at the cathode. I

2. In a process for making iron powder, electrolyzing an aqueous acid solution consisting essentially of ferrous perchlorate dissolved in water maintained at a pH of at least 3, electrodepositing a friable spongy mass of iron at the cathode, and exposing the material deposited at the cathode to a washing and drying operation under oxidizing conditions.

3. A process in accordance with claim 1 wherein the solution is maintained at a pH range of about 3 to about 4.5.

4. A process in accordance with claim 2 wherein the electrolyte employed is ferrous perchlorate and the solution is maintained at a pH range of about 3 to about 4.5.

5. A process in accordance with claim 1, wherein the electrolysis is conducted at a current density of about 35 amperes per square foot of cathode area.

6. A process in accordance with claim 1, wherein the concentration of iron in the electrolytic cell is within the range of about 25 to about grams per liter.

7. A process in accordance with claim 6, wherein a cell voltage within the range of about 1.4 to about 1.6 is employed.

8. In a process for making iron powder, electrolyzing an aqueous acid solution consisting essentially of ferrous perchlorate dissolved in water maintained at a pH of at least 3, electrodepositing a friable spongy mass of iron at the cathode, washing and drying the electrodeposited material under oxidizing conditions, grinding and screening the resulting material, treating the screened material in a reducin atmosphere, and disintegrating the reduced material to iron powder of a size suitable for use as compacts.

9. In a process for making iron powder, electrolyzing an aqueous acid. solution consisting essentially of ferrous perchlorate dissolved in water maintained at a pH of at least 8, electrodepositing a friable spongy mass of iron at the cathode, drying the electrodeposited mass under oxidizing conditions to produce an iron oxide (F6203) content of 10 to 20%, grinding said mass to reduce it to powder, and reducing the oxide to produce iron powder of high purity.

10. A process for electrolytically making iron powder suitable for compacts, which comprises employing an electrolytic cell comprising a stainless steel cathode; an anode of soft steel sheet, and an electrolyte consisting essentially of an aqueous solution of ferrous perchlorate having a concentration of about 35 grams Fe per liter, subjecting the contents of the cell to electrolysis at a pH within the range of about 4.0 to about 4.5, at a current density of about 35 amperes per square foot of cathode area, with a cell voltage within the range of about 1.4 to about 1.6; stripping the cathode deposit, and Washing it twice each time with an amount of water equal to the weight of the deposit; dryin the washed deposit; grinding and screening the resulting material;

r treating the screened material in an atmosphere of hydrogen at temperatures within the range of about 1150 F. to about 1400 R; and disintegrating the reduced material to iron powder of a size suitable for use as compacts.

MAX F. W. I-IEBERLEIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Germany Dec. 5, 1919 Certificate of Correction Patent No. 2,516,515 Jul 25, 1956 MAX F. W. HEBERLEIN It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 4, in the table, last column thereof, line 71, for 0.33 read 0.80; line '73, for 99.60 read 9.9.68; column 5, line 19, extreme right-hand portion thereof,

for the incorrectly printed name now appearing, read Hall;

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 10th day of October, A. D. 1950.

THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

1. IN A PROCESS FOR MAKING IRON POWDER, ELECTROLYZING AN AQUEOUS ACID SOLUTION CONSISTING ESSENTIALLY OF FERROUS PERCHLORATE DISSOLVED IN WATER MAINTAINED AT A PH OF AT LEAST 3 AND ELECTRODEPOSITING A FRIABLE SPONGY MASS OF IRON AT THE CATHODE. 